The present invention relates to a system for verifying access authorization to a vehicle, having a vehicle-based transceiver unit which has at least two transmitting/receiving antennas spaced a distance apart in or on the vehicle, and having a transponder unit which is set up for detection of its presence and for exchanging a predetermined identification request/response sequence through or with the transceiver unit within a predetermined detection area in or around the vehicle.
Such access authorization verification systems, i.e., passive entry systems, are available from various suppliers. They operate bidirectionally and determine the position of persons authorized or approved for a vehicle by detection of a transponder signal for interaction with it within a valid detection area.
In such an access authorization verification system as illustrated in the accompanying FIG. 1, for example, predetermined detection areas A, B and C are provided, within which a transponder T must be located, usually on the right and left sides and at the trunk of vehicle F as well as in the entire interior area of the vehicle. These detection areas A, B and C are determined by the antenna characteristic of antennas mounted on the vehicle, connected to a transceiver unit E, and by the delivery/reception of the transmitting/receiving power of the transponder and the vehicle antennas. A disadvantage of this is that large, complex and expensive vehicle antennas are needed for this purpose. The power output of transponder T may also vary as a function of the temperature and the prevailing battery voltage of the transponder.
In addition, a great effort must be expended in protecting against transponder attacks to prevent unauthorized access to the vehicle.
A device described in German Published Patent Application No. 198 02 526 (assigned to Robert Bosch GmbH) for verifying access authorization involves a time determination, which determines a period of time beginning with transmitting the identification request until receiving the response signal. The period of time thus determined is compared with a limiting value, and access authorization is enabled only if this period of time is less than the limiting value.
The object of the present invention is to avoid the use of large and expensive vehicle antennas on such an access authorization verification system, to make the antenna structure simpler and less expensive, to improve system reliability and to eliminate the possibility of transponder attacks.
The above object is achieved according to an aspect of the present invention by the fact that the transceiver unit has a calculation/analyzing device which performs the detection of a position of the transponder unit by using a time and/or phase measurement based on a mathematical geometric method in response to a reply signal transmitted by the transponder and received by the at least two transmitting/receiving antennas in response to the identification request.
Due to the time and/or phase measurement method performed in the access authorization verification system according to the present invention, the antenna structures are simplified and made less expensive, a higher transmission power of the transponder is made possible and at the same time transponder attacks are made impossible. It is thus possible to defend against the use of a transmission system by an unauthorized person.
In Europe, a frequency band at 433 MHz is allowed by postal authorities for use of transponders, e.g., for automotive key systems, and in the United States a frequency band at 350 MHz is allowed. The transmitting/receiving antennas used in the present invention may be designed as monopoles for these frequencies. These are linear antennas having a ground reference on one side which may be mounted, for example, on the right and left in the rear window and on one side of the front window. The length is approximately 17 cm for the European market and approximately 24 cm for the United States market.
To reduce the size of the antennas and increase the bandwidth allowed by postal authorities, the frequency may be increased with the in the system according to the present invention. Approved ISM frequency bands at 1.25 GHz and 2.4 GHz are preferred here.
According to the present invention, two or even three (which further increases system reliability) transmitting/receiving antennas may be provided on the vehicle.
The calculation and analyzing device of the transceiver unit is set up to measure on at least two transmitting/receiving antennas a transit time difference between transmitting an identification request by a transmitting/receiving antenna and detection of the receipt of the receipt of the identification response transmitted subsequently by the transponder unit. With this measured transit time difference, it is possible to determine the transponder position relative to an unambiguously defined vehicle zero point by using the mathematical geometric method proposed according to the present invention. Internal signal transit times, which are assumed to be constant and of equal length for processing the identification response received by the transceiver unit from the various transmitting/receiving antennas may be used. Likewise, the transponder-internal transit time from reception of the identification request until transmission of the identification response may also be assumed to be constant.
The accuracy, i.e., resolution, during the position determination may be further improved if the calculation and analyzing device also executes a position determination based on a phase analysis in addition to the position determination based on the transit time difference. The phase difference of the identification response received at the multiple transmitting/receiving antennas in response to the transmission of the identification request is determined. The phase measurement is usable to advantage in particular at frequencies in the low megahertz range because wavelength xcex is correspondingly large here. A high-precision time measurement is advantageously unnecessary.
A very advantageous effect of the time measurement method is that the electromagnetic waves travel 30 cm in 1 ns, i.e., a distance of 15 cm within the atmosphere. Access to the vehicle is denied if the measured transit time is greater than a predetermined maximum time, which is composed of the transit time for the distance and the internal transit times.